A Schottky diode is a unipolar device using electrons as carriers, which is characterized by high switching speed and low forward voltage drop. The limitations of Schottky diodes are the relatively low reverse voltage tolerance and the relatively high reverse leakage current. The limitations are related to the Schottky barrier determined by the metal work function of the metal electrode, the band gap of the intrinsic semiconductor, the type and concentration of dopants in the semiconductor layer, and other factors. In contrast to the Schottky diode, a P-N junction diode is a bipolar device that can pass more current than the Schottky diode. However, the P-N junction diode has a forward voltage drop higher than that of the Schottky diode, and takes longer reverse recovery time due to a slow and random recombination of electrons and holes during the recovery period.
A typical device of Schottky diode device with MOS trench has been disclosed by U.S. Pat. No. 5,365,102. Please refer to FIGS. 1A-1F illustrating the manufacturing method of forming the trench MOS barrier Schottky rectifier (TMBSR). Firstly, a substrate 11 with an N-type epitaxial layer 12 grown thereon is provided. Then, a multilayered stack of a pad oxide layer 13, a mask nitride layer 15 and a photo-resist layer 17 is formed on the N-type epitaxial layer 12. The pad oxide layer 13 may relieve interlayer stress between the N-type epitaxial layer 12 and the mask nitride layer 15 (FIG. 1A). A photolithography and etching step is performed to partially remove the mask nitride layer 15, the pad oxide layer 13 and the N-type epitaxial layer 12 so as to form discrete mesas 14 and trenches 20 defined between the mesas 14 (FIG. 1B). A thermal oxide layer 16 is then formed on the trench bottoms and trench sidewalls (FIG. 10). After the remaining portions of the mask nitride layer 15 and the pad oxide layer 13 are removed (FIG. 1D), an anode metal layer 18 is subsequently plated thereon (FIG. 1E). The anode metal layer 18 is then subjected to a metal patterning step, wherein a Schottky barrier contact is formed on the interface between the anode metal layer 18 and the mesa 14 of the N-type epitaxial layer 12. At last, a backside grinding step is conducted on the backside of the wafer and a cathode electrode 19 is formed thereon (FIG. 1F). The process of manufacturing the TMBSR is thus basically completed.
The TMBSR made by the aforementioned method has a low forward voltage drop but a high reverse leakage current. If a lower reverse-biased leakage current is desired, one solution is to choose a metal electrode with a higher work function to reduce the reverse leakage current. However, in this context the forward voltage drop of the TMBSR is increased. Accordingly, there is a trade-off between the forward voltage drop and the reverse leakage current.
An alternative way in reducing the reverse leakage current is to deepen the trenches so as to increase the length of the pitch-off channel to inhibit the reverse leakage current. Nevertheless, such device cannot resist high reverse voltage, unless the thickness of the N-type epitaxial layer 12 is increased to improve the reverse voltage tolerance. In conclusion, Schottky diode is not suitable for high power application. Current commercial Schottky diode device has a reverse voltage tolerance less than 600 V. In fact, so-called 600 V Schottky diode device consists of two TMBSRs connected in series and each has a reverse voltage tolerance of 300 V, which leads to a higher forward voltage drop. Thus, it is a challenge to design a diode device with high reverse voltage tolerance (e.g. higher than 600 V), low forward voltage drop, low reverse leakage current and fast reverse recovery time. Since Schottky diode device has its limitation, a planar P-N junction device may be used to resist high reverse voltage. If the reverse recovery time and the forward voltage drop of the P-N junction device can be improved, the planar P-N junction device can not only be benefited by the advantages identical to that of a Schottky diode device, but also can reduce reverse leakage current and withstand high reverse voltage.